1. Field of the Invention
The present invention relates to a method of manufacturing a photodiode. More particularly, the present invention relates to a method of manufacturing a photodiode that utilizes a chemical vapor deposition process instead of an ion implantation process to form the depletion region of a P-N junction.
2. Description of Related Art
Due to the rapid progress in the electronic industry, manufacturing techniques from different areas such as data transmission, consumer recreation and mobile communication are often integrated together to provide a multi-media function. In particular, multi-media imaging techniques are now mostly mature and have a wide range of applications. As the need for processing images continues to increase, photosensitive chips are once again in great demand in the market. The two most common types of photosensitive chips in the current market include the charge-coupled device (CCD) and the complementary metal-oxide-semiconductor (CMOS) image sensor.
Although most CCD image sensor has a higher immunity against noise interference and a better image quality, it has a lower response speed to external change and incapable of integrating with other system support chip. On the other hand, the CMOS image sensor is formed by using a CMOS fabrication technique. Hence, the CMOS image sensor not only requires less power to operate than a CCD chip, but can also integrate with other MOS devices. Furthermore, as CMOS fabrication technique matures, production cost is also lowered. Nowadays, CMOS image sensors have found a range of applications inside various cost sensitive communication and consumer electronic products.
A typical CMOS image sensor mainly includes a photodiode and a metal-oxide-semiconductor (MOS) transistor. The photodiode has a P-N junction with a depletion region that is sensitive to illumination such that a current passing the photodiode can be used as a signal indicating the intensity of illumination as well as the background noise when the photodiode is in total darkness. Through the signal/noise ratio of the current, the variation of external light intensity can be gauged.
FIG. 1 is a schematic cross-sectional view of a portion of a conventional photodiode. As shown in FIG. 1, the photodiode has a silicon substrate 102 having a P-well 104 and a photosensitive area 106 thereon. The photosensitive area 106 is surrounded by a shallow trench isolation (STI) structure 108. Furthermore, an N-doped region 110 is formed above the P-well 104 within the photosensitive area 106 by performing an ion implantation process. Because the P-N junction at the interface between the P-well 104 and the N-doped region 110 produces a depletion region 112, the depletion region 112 can be used as a sensing region for detecting the intensity of external light. However, the aforementioned photodiode 100 has a narrow depletion region 112 and the depletion region 112 is located at a definite depth below the surface so that the signal/noise ratio is rather low. Ultimately, photodiode is unsuitable for sensing light at a shorter wavelength.
FIG. 2 is a schematic cross-sectional of a portion of another conventional photodiode as disclosed in U.S. Pat. No. 6,566,722. As shown in FIG. 2, a P-type substrate 202 is provided and then a P-type epitaxial silicon layer 204 is formed over the substrate 202. Thereafter, a photolithographic and an ion implantation process are carried out in sequence to form a plurality of first N-doped regions 206 on the P-type epitaxial silicon layer 204. Finally, another photolithographic and ion implantation process are carried out in sequence to form a second N-doped region 208 over the P-type epitaxial silicon layer 204 and the first N-doped regions 206. After performing the aforementioned steps, a plurality of trench-shaped first N-doped regions 206 is formed in the P-type epitaxial silicon layer 204 so that the contact area between the first N-doped regions 206 and the P-type epitaxial silicon layer 204 is increased. With an increase in the area of the depletion region 210, the photodiode can have higher signal sensitivity. In addition, through the second N-doped region 208 on the P-type epitaxial silicon layer 204, sensitivity of the photodiode with respect to light of a shorter wavelength is enhanced. However, an ion implantation process is still deployed in the fabrication of the photodiode so that the density of dopants within the depletion region 210 may fluctuate and affect the sensing accuracy.
FIG. 3 is a schematic cross-sectional of a portion of another conventional photodiode as disclosed in U.S. Pat. No. 6,611,037. As shown in FIG. 3, a P-type substrate 302 is provided and then trenches 304a and 304b are formed in the substrate 302. Thereafter, an ion implantation process is carried out to form an N-doped region 306 in the interior sidewalls of the trenches 304a and 304b as well as the P-type substrate 302 between the trenches 304a and 304b. Next, an isolation layer 308 and a conductive layer 310 are sequentially formed over the N-doped region 306. The interface between the N-doped region 306 and the P-type substrate 302 produces a P-N junction having a trench-like depletion region 312 capable of increasing overall sensitivity of the photodiode. However, due to the shape of the trench, a multiple ion implantation operations with the ion beam in each operation set to a different angle must be carried out to form a uniform N-doped region 306. Ultimately, both the processing time and the production cost of the photodiode are increased.